When i am on park or neutral, I don't get the vibration, only when I'm on drive or when I'm driving. Even when I let go of the gas pedal while I'm going over 10 kph I get this vibration.
I've read online that it's probably the rear U-joint, but I want to make sure before taking it apart.
The vibration peaks when I'm going 40 kph but it is always present when I'm on drive
So, am I right in my assumption that it is the rear U-joint?

 

Jeep parked on a flat surface, wheels chocked to prevent it from moving, key on acc so you can manipulate the trans shifter - put the jeep in neutral and crawl under there. With the jeep in neutral you will be able to twist the driveshaft and watch the ujoints for free play from the ujoint caps. If there is a fine dusty or rusty look to it then consider them worn out.